1. Field of the Invention
The present invention relates to apparatus for transmitting a radio frequency signal to a medical device implanted in an animal, and more particularly to cardiac pacing devices in which the radio frequency signal causes the implanted device to deliver energy to cardiac tissue for the purpose of stimulating contractions.
2. Description of the Related Art
A remedy for people with slowed or disrupted natural heart beating is to implant a cardiac pacing device. A cardiac pacing device is a small electronic apparatus that stimulates the heart to beat at regular intervals. That device consists of a pulse generator, implanted in the patient's chest, which produces electrical pulses that stimulate heart contractions. Electrical wires extend from the pulse generator to several electrodes placed adjacent specific muscles of the heart, which when electrically stimulated produce contraction of the adjacent heart chambers.
It is quite common that the wires extend through arteries or veins which enter the heart so that the electrodes can be placed in the muscle of the heart chamber requiring stimulation. The wires typically extend for some distance through the arteries or veins and may pass through one or two heart valves. In other patients, patch electrodes are placed on the exterior heart surface with wires extending through tissue to the pacing device. With either type of wire placement, it is important that the electrodes be attached to the proper positions on the heart to stimulate the muscles and produce contractions. Thus, it is desirable to properly locate the electrodes for maximum heart stimulation with minimal adverse impact to other physiological functions, such as blood circulation.
More recently wireless pacing devices have been proposed, such as the one described in U.S. Pat. No. 6,445,953. With this type of device, a radio frequency (RF) signal is transmitted from a conventional pacing circuit to stimulator devices placed on the heart at locations where stimulation is to occur. For example, the stimulator device can be mounted on a stent that is implanted in a blood vessel of the heart. The radio frequency signal activates the stent which applies an electrical stimulation pulse to the heart tissue. Electrical power for stimulating the heart is derived from the energy of the radio frequency signal.
One of the difficulties in this wireless system is ensuring that a maximum amount of the RF energy is received by the stimulator device. In the case of a stent, the antenna is a coil located on a cylindrical surface and receives the greatest amount of energy from an electromagnetic field oriented in a direction through the turns of the coil. However, since the stent can be implanted in different orientations in the patient's body and the orientation of the transmitter antenna similarly varied, it is difficult to ensure that the electromagnetic field from the RF signal will be properly oriented with respect to the stent antenna.